Liquid Fuel Bypass Valve


Thread Starter


We have a dual fuel GE 6B DLN1 combustion Turbine with MARKV controls. We have changed out the Servo on the liquid fuel bypass valve and want to check it if polarity is correct on the installation of the new servo. Is there a way to stroke this valve? We have it in "auto-calibration" page but no luck. We are able to stroke the SRV, CV, Splitter valve and IGV servos and perform the check of removing 2 out of 3 servo command currents. They work. The LFBV has no LVDT's or any means of showing position except for visible shaft movement.

About the only way to do this is to unbolt the LFBV from the piping so that one can see the valve plug and then check servo polarity.

Unfortunately, when one changes the reference to the LFBV (either from AutoCalibrate or from a Demand Display) one is telling the LFBV to increase fuel flow--and since there is no feedback from the Liquid Fuel Flow Divider the valve plug will move to the fully closed position trying to increase the feedback from the flow divider's speed pick-ups.

BUT, you can do this under the control of individual processors (<R>, <S> and <T>). With the valve plug visible you can increase the reference to the LFBV above 0, to say 3 or 5 or 10 (using the Manual Feature of AutoCalibrate, or from the LFBV Demand Display). The plug should move to the fully close position (it doesn't travel very far!). and then when you put in a very small negative reference, say, -3 or -5 or -10 it should move back to the fully open position. If, when you increase the reference in the positive direction the valve plug doesn't move from the fully open position, but when you increase the reference in the negative direction the valve moves to the fully closed position then the polarity is backwards for that coil and needs to be changed.

Do this by disconnecting one of the servo output leads for <S> and one for <T> to test <R>. Then connect <S>'s lead again and disconnect one of <R>'s servo coil leads to test <S>. Finally, connect <T>'s disconnected lead and disconnect one of <S>'s to test <T>.

This is messy (liquid fuel will leak out of the open piping flanges), but it about the only way to do this test when the valve plug and/or stem are not visible and there are no LVDTs. One must realize that without any feedback from the Liquid Fuel Flow Divider the valve plug will never hover in any mid-range position--it will just go full closed when there's a positive reference, and full op when there's a negative reference. (The LFBV closes to increase liquid fuel flow-rate to the turbine, and opens to decrease liquid fuel flow-rate to the turbine--because it's a bypass valve.)

I've also done it by putting the machine in FIRE mode and disabling the spark plugs/ignitors and "false firing" the machine (selecting START knowing the spark plugs/ignitors are disabled). Liquid fuel will flow into the turbine, but it can't ignite because if the turbine is cold and the spark plugs/ignitors are disabled. This will require at least three attempts (if the servo polarities are correct) and will require a longer than normal purge by putting the unit on CRANK after the testing is complete. If the polarity or each servo coil is correct, there will be fuel flow during firing and the fuel flow-rate will be controlled to match the reference. If the polarity is incorrect there will be no flow during firing.

A lot of sites get extremely nervous when doing this, and rightfully so if they're not familiar with the unit and how it works. And, it consumes liquid fuel--which is usually more expensive than gas fuel.

But, these are the only two ways I know of to verify the polarity of the currents being applied to the servo coils for a Frame 5 or Frame 6B GE-design heavy duty gas turbine with closed valve and no LVDTs. Neither is very easy, and both are time-consuming and one consumes liquid fuel.

Hope this helps!